The present invention relates to a filter for microwave and millimeter waves which passes desired frequency bands and interrupts unnecessary frequency bands. More particularly, the present invention relates to a filter which saves space when employed as a waveguide and which enables simplification when built into a microwave circuit.
As for a band-pass filter employing a cavity, there has been a known filter whose arrangement is shown in schematic view in FIG. 5. In this arrangement, an iris plate 32 is inserted into a rectangular waveguide 31, and the iris plate 32 acts as an inductive resonant window (reactance element) and a plurality of pairs of resonant windows are provided along the tube axis direction. A cavity resonator is formed in the space between two adjacent resonant windows in the longitudinal direction and the tube walls of the rectangular waveguide 31. The iris plate 32 is provided to obtain desired band characteristics through design choices, such as the distance between adjacent resonant windows in the longitudinal direction or the size of the resonant window opening (distance between a pair of iris plates). In the example shown in FIG. 5, the iris plate 32 is illustrated in a perspective view with three layers of cavity resonators being connected in series therein. By arranging a filter by connecting a plurality of cavity resonators, it is possible to arrange a filter which enables one to obtain a sharp attenuation of a rejection band, in other words, a filter which enables one to obtain large attenuation at a rejection frequency being close to the pass band, as compared with an arrangement of a filter employing one cavity resonator (for example, see Japanese Unexamined Patent Publication No. 230603/2001).
In order to improve filter characteristics, multiple layers of cavity resonators are required to be connected. However as shown in the above-mentioned arrangement, the cavity resonators are connected in a linear manner and it become longer with an increased number of layers, since one cavity resonator has to have ½ the inside tube wavelength. Accordingly, there has been a problem in that it becomes extremely difficult to insert the cavity resonators into a microwave circuit. In addition, it is impossible to build the multiple layers of iris plates into the waveguide, and the waveguide is required to be connected after being divided along a center line in an axis direction or separating the wider surface and connected with the iris plates.
On the other hand, as shown in FIG. 6, there is a known filter arranged through forming multiple layers of cavity resonators being divided by iris 34 through forming a waveguide groove pattern 35 having iris 34 in a conductive casing 33 and covering the surface of the conductive casing 33 by a cover not shown in figures, and the filter also has a desired frequency characteristics in such a manner as the above-mentioned example (for example, see Japanese Unexamined Patent Publication No. 209406/2003). In an example shown in the last cited reference, there is described a filter being capable of passing only the desired frequency characteristics while bending the propagation direction of an electromagnetic wave at 90°. It is disclosed that corners of the cavity are formed in a special shape in order to bend the propagation direction of an electromagnetic wave at 90°.
As mentioned above, multiple layers of resonators are required to be connected through inductive resonant windows for improving the filter characteristics with respect to the conventional filters having a waveguide form. Therefore, the layout of the microwave circuit has to be modified, since multiple layers of resonators become longer and are difficult to be inserted into the existing circuit (microwave circuit). In addition, there have been problems of leakage of radio waves or characteristic deterioration in case there is a space between the filter and existing circuit, and there have been problems of characteristic deterioration in the case where there is a deformation of the microwave circuit by forcible fastening to avoid the space. The same problems are applied to the above-mentioned arrangement in FIG. 6 in which the propagation direction of the electromagnetic wave is bent at 90°. There is no problem in the case where the microwave circuit is assembled from the beginning or the microwave circuit includes one block. However, in the case where the microwave circuit is already built-in there is a problem in replacing filters in that a different filter cannot be built-in for improving the filter characteristics.
In order to obtain the sharp cut-off frequency characteristics of the filter or large attenuation of unnecessary frequency band as mentioned above, the number of cavity resonators is required to be increased to include multiple layers. However, there has been a problem that space efficiency is extremely reduced, since multiple layers of cavity resonators become extremely long in the case where they are connected in a straight line, and a large area is occupied even if the propagation direction is bent at 90°.
The present invention is provided to solve the above-mentioned problems. An object of the present invention is to provide a filter with an arrangement of waveguide form, which can be simply replaced with a filter having high performance without changing the layout of an existing microwave circuit, and certainly connected to filters without causing deterioration of characteristics.
Another object of the present invention is, even if the waveguide is divided into a body and cover, to provide a filter, which enables prevention of leakage of electromagnetic waves at a connecting part through providing the connecting part where the electric field is weakest.